Glycophosphate (aka glyphosphate / Roundup) inhibits an enzyme essential to plant growth. It is not very effective against Japanese Knotweed's underground rhizomes but will kill foliage thus starving the rhizomes over time.
An environmentally friendlier herbicide that kills foliage on contact is 15-20% vinegar solution (handle with care), which is non-toxic, fully bio-degradable and causes no river/water-table pollution or human/animal health concerns.
Recent research by Swansea University into knotweed extermination can be accessed from this page near the bottom:
Swansea University scientists lead the way in tackling Japanese knotweed
or directly at:
Optimising physiochemical control of invasive Japanese knotweed